When a vehicle travels at a high speed, a lift force is generated in a vertical direction of the vehicle, and a grip force of a tire is reduced because of an aerodynamic force that flows along a surface of the vehicle, thereby adversely affecting traveling stability.
Since an upper portion of the vehicle has a curved shape and a lower portion of the vehicle is a plate shape, a flow velocity of air flowing along the upper portion of the vehicle is faster than a flow velocity of air flowing along the lower portion of the vehicle, and a pressure difference between the upper portion and the lower portion of the vehicle is generated. Thus, the lift force is generated by the pressure difference.
A fixed spoiler disposed at an upper portion of a trunk lid has been developed for reducing the lift force. The fixed spoiler disposed at the upper portion of the trunk lid may provide a compensation operation of the lift force when the vehicle travels at a high speed, such that the traveling stability and stable braking force may be provided by increasing a frictional force of the tire.
However, in the case of using the fixed spoiler, air resistance flowing along the surface of the vehicle is increased when the vehicle travels at a low speed, and thus, fuel efficiency and traveling performance are deteriorated.
In order to solve the problems, an adjustable spoiler that is extended when the vehicle speed is higher than a reference vehicle speed for reducing the lift force and increasing the grip force has been developed. The extension of the conventional adjustable spoiler is performed according only to a vehicle speed condition.
Even though a sufficient grip force of the tire is generated in a case that the vehicle weight is increased by the number of passengers or the amount of loaded freight, the adjustable spoiler is extended when the vehicle speed condition is satisfied, and thus, a drag force due to the aerodynamic force and rolling resistance of the tire due to an unnecessarily tire load are generated. The drag force and the rolling resistance of the tire may decrease fuel efficiency.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.